I've been looking into building a in-the-ear type headset using low-end
earphones, following the example given at
http://www.canardzone.com/members/ni...ss_headset.htm

I bought the Phillips ANR headphones from Target. I've got Jim Weir's
impedance matching circuit article describing how to use the audio
transformer, so the headphone portion is in the bag.

The hangup is with the microphone. I found some discussions where Jim
said that an amplifier is needed to use an electret element. Well, I
picked up Item# 270-090 from Radio Shack today.
http://www.radioshack.com/product/in...rt&tab=summary

It has a wiring diagram on the back of the package that seems to
indicate that the single transistor amplifier is already built into the
package. I was going to post here and ask if that was in fact what it
indicates, and inquire whether it would work with the RST-443 intercom.

Then I got antsy and just went and tried it. With only two leads, you
know I had to hook it up backwards and feel disappointed for a minute
until I figured out to turn the leads around. Then it was "WOOHOO!!"
all the way.

I'm now well on my way to having the equivalent of a $500, in-the-ear
headset, and it's only costing me about $50. (Testing is incomplete,
but others have reported resounding success.)